The present invention is directed to a variable valve actuation system and, more particularly, to a system and method for controlling a variable valve actuation system to operate an engine.
Emission reductions and fuel efficiency are competing concerns in the design and operation of an internal combustion engine, such as, for example, a diesel, gasoline, or natural gas engine. Accordingly, a significant amount of research and development work is being directed towards reducing the emissions while maintaining or improving the fuel efficiency of these types of engines. Any increase in fuel efficiency will directly translate to a reduction in the fuel costs associated with operating the engine along with the production of carbon dioxide.
Oxides of nitrogen (xe2x80x9cNOxxe2x80x9d) are one constituent of engine emissions that researchers are trying to reduce. NOx production is generally proportional to temperatures of combustion and volume of excess air. However, fuel efficiency is also generally proportional to these same factors. Conventional NOx reduction techniques include increasing the mass of inert matter, such as water or recirculated exhaust gas, in a combustion chamber prior to combusting a fuel and air mixture. These measures may reduce the combustion temperatures and the generation of NOx, but may also reduce the fuel efficiency.
One approach to compensate for a reduction in fuel efficiency involves increasing the pressure of the intake air provided to the engine and improving control over the flow of gases into and out of the engine. This may be accomplished by placing a turbocharger in the intake air flow path and by modifying the typical engine valve actuation system to provide flexibility in the actuation timing of the intake and exhaust valves. In this manner, the flow of gases to and from the engine may be tailored to meet the particular operating conditions of the engine and thereby improve the fuel efficiency of the engine.
The engine valves in an internal combustion engine are typically driven by a cam arrangement that is operatively connected to the crankshaft of the engine. The rotation of the crankshaft results in a corresponding rotation of a cam that drives one or more cam followers. The movement of the cam followers results in the actuation of the engine valves. The shape of the cam governs the timing and duration of the valve actuation.
An engine may, however, include a variable valve actuation system, such as described in U.S. Pat. No. 6,237,551 to Macor et al., issued on May 29, 2001. In this type of system, the cam arrangement is configured to hold the engine valves open for a certain period of time and an auxiliary valve is included to selectively disengage the cam assembly. This allows the engine valves to be closed earlier than provided by the timing of the cam assembly and improves the control over valve actuation timing.
The improved control provided by a variable valve actuation system may allow for gains in fuel efficiency. The variable valve actuation system may be operated to selectively implement a variation on the typical diesel or Otto cycle during the operation of the engine. For example, the intake valves may be controlled to implement a xe2x80x9clate intakexe2x80x9d type Miller cycle. In a late intake Miller cycle, the intake valves are opened for the intake stroke and held open for a portion of the compression stroke of the piston.
In addition, the exhaust valves may be controlled to implement an engine braking cycle. In an engine braking cycle, the exhaust valves are opened when the piston approaches a top-dead-center position of the compression stroke to release the air compressed during the combustion stroke. By releasing the compressed air instead of initiating combustion, the engine is operated as an air compressor instead of a power generator. In this manner, the engine may be operated to dissipate the kinetic energy of a moving vehicle to help slow the vehicle.
When, however, a turbocharger provides pressurized air to the internal combustion engine, a variation from the cam driven valve actuation timing may result in excessive compression ratios in the combustion chambers. If the maximum compression ratio in the combustion chambers consistently exceeds a certain level, the high compression ratios may damage any of several engine components. For example, the high compression ratios may damage the valve actuation assembly and/or the engine braking components. In addition, the high compression ratios may damage the engine gasket. This type of damage may result in expensive and time-consuming maintenance to repair.
The system and method of the present invention solves one or more of the problems set forth above.
In one aspect, the present invention is directed to a method of controlling a valve actuation system for an engine. An intake cam assembly is operated to move an intake valve between a first position where the intake valve blocks a flow of fluid relative to a cylinder and a second position where the intake valve allows a flow of fluid relative to the cylinder. An exhaust cam assembly is operated to move an exhaust valve between a first position where the exhaust valve blocks a flow of fluid and a second position where the exhaust valve allows a flow of fluid. An engine brake is operated to selectively move the exhaust valve from the first position towards the second position when a piston is at or near a top-dead-center position of a compression stroke. At least one operating parameter of the engine is sensed. A desired intake valve actuation period is determined based on the at least one operating parameter. A valve actuator is engaged with the intake valve to prevent the intake valve from returning to the first position in response to operation of the intake cam assembly. The valve actuator is released to allow the intake valve to return to the first position at the end of the determined valve actuation period.
In another aspect, the present invention is directed to an intake valve actuation system for an engine that includes an intake valve moveable between a first position where the intake valve prevents a flow of fluid and a second position where the intake valve allows a flow of fluid. An intake cam assembly is connected to the intake valve to move the intake valve between the first position and the second position. An exhaust valve is moveable between a first position where the exhaust valve prevents a flow of fluid and a second position where the exhaust valve allows a flow of fluid. An exhaust cam assembly is connected to the exhaust valve to move the exhaust valve between the first position and the second position. A brake actuator is selectively operable to engage the exhaust valve to move the exhaust valve from the first position towards the second position. A valve actuator is selectively operable to engage the intake valve and prevent the intake valve from returning to the first position. A sensor is operable to sense an operating parameter of the engine. A controller is operable to determine a valve actuation period based on the sensed operating parameter of the engine and to actuate the valve actuator to prevent the intake valve from returning to the first position for the determined valve actuation period.